Page 91 - Biofuels Refining and Performance

74 Chapter Three

amounts of fats and fibers. The compositions of the crop roots are almost
identical to those of the cereals on a dry basis, but the water content of
the roots is usually 70–80%. The exact composition of each raw mate-
rial depends on the type and variety of materials used and can be found
in literature (e.g., [7]). Starch from these materials is used as a carbon
and energy source, and part of the proteins as a nitrogen source, by the
microorganisms.
Starch contains two fractions: amylose and amylopectin. Amylose,
which typically constitutes about 20% of starch, is a straight-chain
polymer of -glucose subunits with a molecular weight that may vary
from several thousands to half a million. Amylose is a water-insoluble
polymer. The bulk of starch is amylopectin, which is also a polymer of
glucose. Amylopectin contains a substantial number of branches in
the molecular chains. Branches occur from the ends of amylose seg-
ments, averaging 25 glucose units in length. Amylopectin molecules are
typically larger than amylose, with molecular weights ranging up to
1–2 Mg. Amylopectin is soluble in water and can form a gel by absorb-
ing water.
For ethanol production, hydrolysis is necessary for converting starch
into fermentable sugar available to microorganisms. Traditional con-
version of starch into sugar monomers requires a two-stage hydrolysis
process: liquefaction of large starch molecules to oligomers, and sac-
charification of the oligomers to sugar monomers. This hydrolysis may
be catalyzed by acid or amylolytic enzymes.

3.4.2 Acid hydrolysis of starch
Acid hydrolysis is an old process still applied in some ethanol industries.
Sulfuric acid is the most commonly applied acid in this process, where
starch is converted to low-molecular-weight dextrins and glucose [8].
Main advantages of this process are rapid hydrolysis and less cost for
catalyst, compared to the enzymatic hydrolysis. However, the acid
processes possess drawbacks including (a) high capital cost for an acid-
resistant hydrolysis reactor, (b) destruction of sensitive nutrients such
as vitamins present in raw materials, and (c) further degradation of
sugar to hydroxymethylfurfural (HMF), levulinic acid, and formic acid,
which lowers the ethanol yield and inhibits the fermentation process [9].
The acid hydrolysis process can be performed either in batch or in con-
tinuous systems. Dilute-acid hydrolysis can also be used as a pretreat-
ment for enzymatic hydrolysis. It is common to soak the starch or starchy
materials in the dilute acid prior to enzymatic hydrolysis, then to con-
tinuously pass it through a steam-jet heater into a cooking tube (called
a jet cooker or mash cooker) with a plug flow residence time for a couple
of minutes, and then subject it to enzymatic hydrolysis.